Adding an email address to OS X Mail so that you can send mail from it is simply a matter of adding it to the list of email addresses handled by your email account.

1. First, select Preferences… from the Mail menu.

   

2. The Preferences pane will open. Click on Accounts.

   

3. Add the email address to the Email Address box. Email addresses should be separated by a comma and a space.

   

4. Close the Preferences pane with the red bobble in the top left corner of the window. You will be prompted to save the changes. Click Save.

   

5. You will now be able to choose which address to send mail from when composing a new message.